A random mutation that suffers no evolutionary pressure is usually spread randomly across the population. Eye color for example.
As opposed of something that gives an advantage (like lactose tolerance or falciform anemia) which usually spreads quickly, while those that don't have that advantage died out.
When our ancestors climbed down from their trees, they also started eating windfall. Ripe fruits tend to ferment. This is the reason why most carnivores can’t metabolize alcohol, while humans and other herbi-/omnivores can.
The evolutionary pressure was to be able to eat more.
As opposed of something that gives an advantage (like lactose tolerance or falciform anemia) which usually spreads quickly, while those that don't have that advantage died out.